MUNICH -- The United States is prepared to hold direct talks with Iran in the standoff over its nuclear ambitions, Vice President Joe Biden said Saturday -- but he insisted that Tehran must show it is serious and Washington won't engage in such talks merely "for the exercise."

During a trip to an international security conference in Germany, Biden also addressed Syria's civil war. He met with top Syrian opposition leader Moaz al-Khatib and with Foreign Minister Sergey Lavrov of Russia, a longtime ally of Syrian President Bashar Assad. Separately, al-Khatib met with Lavrov for the first time, offering a glimmer of hope for stalled diplomatic efforts to resolve the conflict, though the Russian minister later sounded skeptical.

Washington has indicated in the past that it's prepared to talk directly with Iran, and talks involving all five permanent members of the U.N. Security Council plus Germany have made little headway while several rounds of international sanctions have cut into Iran's oil sales and financial transactions.

Last month Iran, in a defiant move ahead of new talks expected soon with the six powers, announced plans to vastly increase its pace of uranium enrichment. That can be used to make both reactor fuel and the fissile core of warheads.

Biden told an international security conference that "there is still time, there is still space for diplomacy backed by pressure to succeed." He did not specify any timeframe.

He insisted that "the ball is in the government of Iran's court" to show that it's negotiating in good faith.

The U.S. has long made clear that it is prepared to meet directly with the Iranian leadership, he added -- "that offer stands but it must be real and tangible and there has to be an agenda that they're prepared to speak to."

"We are not just prepared to do it for the exercise," Biden told the Munich Security Conference.

Russia's Lavrov, whose country is a key player in the six-nation talks with Iran, said it was important to offer Iran clear incentives to resolve the nuclear standoff. "We have to convince Iran that it is not about the regime change," he said.

Iran insists it does not want nuclear arms and argues it has a right to enrich uranium for a civilian nuclear power program, but suspicion persists that the real aim is nuclear weapons. Iran's foreign minister, Ali Akbar Salehi, is to address the conference Sunday.

While Russia and the U.S. have worked together on Iran, their differences over Syria were on display again at the conference in Munich, an annual gathering of top security officials.

In his speech, Biden stressed the conviction of the U.S. and many others that "President Assad -- a tyrant hell-bent on clinging to power -- is no longer fit to lead the Syrian people and he must go." He said that "the opposition continues to grow stronger."

He later held separate meetings with Lavrov, international peace envoy Lakhdar Brahimi, and Syria's top opposition leader, Moaz al-Khatib.

On the sidelines of the conference, Lavrov in turn met for the first time with al-Khatib -- who in December rejected a previous Russian invitation for talks, Russian news agency ITAR-Tass reported.

On Friday, al-Khatib said he was willing to sit down for talks with Assad's government to "ease the pain of the Syrian people."

Lavrov welcomed that initiative, and said on his flight back to Moscow that Russia and the Syrian opposition plan to stay in regular contact, Russian state news agency RIA Novosti reported.

"This is an important step, given that the coalition was created on the platform of rejecting talks with the regime," he said. "But I think that realism will prevail. Naturally, this does not guarantee that dialogue will start, as the opposition does not have a negotiating team and there are many different groups to agree on a single delegation."

At the conference, Biden told an audience including Lavrov that despite differences, "we can all agree on the increasingly deep plight of the Syrian people and the responsibility of the international community to address that plight."

But Lavrov fired back that "there are a lot of question marks about the Western approaches to those developments," in the region, and questioning when it is "permissible to cooperate with regimes and when is it legitimate to argue for their removal."

Lavrov also suggested Biden's statement that Assad must go was counterproductive.

"The persistence of those who say that priority No. 1 is the removal of President Assad -- I think it's the single biggest reason for the continued tragedy in Syria."

Asked whether Russia might be prepared to endorse humanitarian corridors protected by air power, Lavrov bluntly replied: "No. Any threat of use of force will be unacceptable."